Riser is a conduit to transfer product between subsea wells and surface facilities. The main fuctions of a riser system includes the following:
The main scope to design a riser system is presented below.
Riser functions similar as a pressure vessel. Riser design begains with material selection and pipe sizing to withstand the inernal or external overpressure. When the pipe diameter is selected for the flow consideration, the pipe wall thickness is designed for internal pressure containment, which is to prevent the pipe burst failure due to the internal pressure. Pipe should also be designed to prevent the collpase and buckling failure due to the external pressure and combined bending.
30 CFR 250 has the following formulation for internal pressure design: \[P = {2\cdot S\cdot t \over D}F\cdot E\cdot T \tag{1}\label{eq1} \] where
30 CFR 250 uses Barlow's Equation to calculate the internal design pressure that a pipe can withstand to its dimensions and the strength of its material. The design factor of 0.6 indicates the the hoop stress due to the internal design pressure should be limited to 60% of yield strength for riser component.
The Federal Regulations codes and industrial standards employ Barlow's Equation to calculate the internal design pressure that a pipe can withstand, considering its dimensions and material strength. A constant hoop stress along the wall thickness is calculated, which should be less than 60% of the yield strength. The required wall thickness based on ASME codes is typically less than that required by CFR codes. ASME codes use the differential pressure, whereas CFR codes consider the internal design pressure only. The formula for riser wall thickness internal pressure design (burst) is presented below in Table 1.
| Riser | Code | Formula | Design Factor F | Pressure |
|---|---|---|---|---|
| DOI Pipelines | 30 CFR 250.1002 | t=PD/(2SEFT) | 0.6 | Internal |
| API RP 1111 | pb=k(S+U)ln(D/Di), p<F*pb | 0.6 | Differential | |
| DOT Pipelines (Liquid) | 49 CFR 195.106 | t=PD/(2SEF) | 0.6 | Internal |
| 30 CFR 250.1002 | t=PD/(2SEFT) | 0.6 | Internal | |
| B31.4 A402.3.2 | t≥pD/(2FS) | 0.6 | Differential | |
| API RP 1111 | pb=k(S+U)ln(D/Di), p<F*pb | 0.6 | Differential | |
| DOT Pipelines (Gas) | 49 CFR 192.105 | t=PD/(2SEFT) | 0.5 | Internal |
| 30 CFR 250.1002 | t=PD/(2SEFT) | 0.6 | Internal | |
| B31.8 A842.2.2 | t≥pD/(2FST) | 0.5 | Differential | |
| API RP 1111 | pb=k(S+U)ln(D/Di), p<F*pb | 0.6 | Differential |
API RP 1111 presents the formula to calculate the pipe burst pressure and set up the 60% of the burst pressure as the limit of the design pressure. The burst pressure formula from pipe testing or can be verified using finite element analysis method (von Mises stress at the outfer reaches the yield strength). It's worth noting that calculations based on CFR using Barlow's Equation are often conservative for thick-walled pipes. A request to use API RP 1111 to determinethe wall thickness is an alternate compliance from requirment of 30 CFR 250 per BSEE NTL 2009-G28.
Pipe collpase pressure are calculated based on pipe material and size. API RP 1111 and API RP 2RD employ the same collapse check formula, differing only in the collapse factor used. API RP 1111 utilizes a collapse factor of 0.7 (limiting the external overpressure to 70% of the collapse pressure) for seamless or ERW pipe, while API RP 2RD adopts a collapse factor of 0.75 for the same pipe types. Both codes apply a collapse factor of 0.6 for cold expanded pipe, such as DSAW pipe.
API RP 1111 sets the limit for combined bending strain and external pressure load. The maximum bending strain in the pipe is compared to the buckling strain under pure bending, and the external overpressure is compared to the collapse pressure.
The riser sizing module calcuates the required wall thickness for internal and external pressures. Based on the selected wall thickness, the riser strength and fatigue performance should be assessed to meet the design requirements.
Riser pipe cross-section weight can be calcuated when the riser pipe sizing is determined. An equivalent section weight includes the weight from the steel, coatings, thermal insluation, and the auxilary equipment such as strakes and buoyancy modules. The section weight can be calculated for different conditions during installation and operation, with various fluid content densities. The pipe cross-section weight is required for the calculation of the riser top tension or payload to the platform. In pipeline design, the pipe cross-section weight is used for the assessment of the pipeline interaction with soil, such as pipeline on-bottom stability.
Buoyancy factor is defined as the ratio of net buoyancy (total up-lift force) to the bare riser's submerged weight.Below is an example illustrating the calculation of the buoyancy factor:
Steel pipe outer diameter: 14-inch
Steel pipe wall thickness: 1-inch (pipe ID 12-inch)
Buoyancy module thickness: 10.5-inch
Buoyancy ID 14-in and OD 35-inch
Buoyancy density: 38.5 lb/ft3
Seawater density: 64 lb/ft3
Steel pipe dry weight: \({\pi \over 4} \cdot {\left( 14^2-12^2 \right) \over 144 } {\cdot 490} = 138.97 \; lb/ft \)
Steel pipe buoyancy: \({\pi \over 4} \cdot {\left( 14^2 \right) \over 144 } {\cdot 64} = 68.42 \; lb/ft \)
Steel pipe submerged weight: \(138.97 - 68.42 = 70.55 \; lb/ft \)
Buoyancy module dry weight: \({\pi \over 4} \cdot {\left( 35^2-14^2 \right) \over 144 } {\cdot 38.5} = 216.07 \; lb/ft \)
Buoyancy module buoyancy: \({\pi \over 4} \cdot {\left( 35^2-14^2 \right) \over 144 } {\cdot 64} = 359.19 \; lb/ft \)
Buoyancy module net buoyancy: \(216.07 - 359.19 = -143.12 \; lb/ft \)
Buoyed pipe submerged weight: \(70.55 - 143.12 = -72.57 \; lb/ft \)
Buoyancy factor: \({143.12 \over 70.55} = 2.03 \)
The riser configuration is calculated using the following equations related to catenary shape:
\[ A_{TDP} = { {H \cdot sin\left(\theta\right)} \over {1-sin\left(\theta\right)} } \tag{1}\label{eqd1} \] \[ L = \sqrt {H^2 + 2\cdot H \cdot A_{TDP} } \tag{2}\label{eqd2} \] \[ X = A_{TDP} \cdot asinh \left({ L \over A_{TDP} }\right) \tag{3}\label{eqd3} \] \[ T_{hangoff} = { {W \cdot H } \over { 1 - sin\left(\theta\right)} } \tag{4}\label{eqd4} \] \[ T_{TDP} = T_{hangoff} \cdot sin\left(\theta\right) \tag{5}\label{eqd5} \]where
The catenary shape is determined using only the hang-off height and top angle. This implies that risers with the same hang-off height and angle on a given platform will have identical configurations (suspended length and horizontal length), regardless of their sizes and weights. In detailed riser finite element analysis, the riser pipe’s axial and bending stiffness are considered. However, for deepwater risers, these stiffness effects are minimal, and the theoretical catenary formula provides an accurate result for riser configuration and tension calculation.
An integrated design of SCR is to develop a riser system to meet the main funcitonal requirements. The technical report serves as the basis for the design of the riser system. The main design parameters have been determined to meet the functional requirements outlined. These design inputs include the following:
For production riser, the study focuses on designing a multi-layer polypropylene (PP) or multi-layer polyethylene (PE) thermal insulation coating system to meet anti-corrosion and insulation requirement. To achive the target overall heat transfer coefficient or U-value, the thickness of GSPP (coating for thermal insulation) is calculated.
The riser section weight will be computed and used for riser top tension calculation. The SCR configuration will be generated for three vessel postions. The bending stress at the outer fiber of the riser pipe is presented.
The riser installability will be assessed for S-lay, J-lay, or reel-lay method. To prevent on-reel buckling, the riser wall thickness will be checked with the minimum reelable wall thickness.
The riser material take-off (MTO) will be provided and the cost will be estimated for material procurement, testing, fabrication, transportation, and installation for three different installation methods.
The design criteria are also presented for riser sizing, coating design, configuration calculation, finite element modeling, strength, fatigue, and interference analysis.
An integrated design of SLWR is similar to SCR. The difference is to install buoyancy modules to the lower section of the riser, to form the lazy wave configuration. The document to summarize the results from an integrated design serves as the basis for the design of the SLWR system.
The buoyancy module properties will be added in the design inputs. The riser buoyed section submerged weight and buoyancy length will be used for the lazy wave configuration. The bend radius and bending stress at the hog bend (buoyancy module section) will be report. The buoyancy module impact on installation is discussed. The required buouancuy module is included in riser MTO and cost esitmate.
The catenary formula indicates that a buoyancy factor of 2.0 provides an ideal lazy wave configuration. This occurs when the submerged weight of the buoyed section equals the submerged weight of the bare pipe but acts in the opposite direction (upward).
An integrated design of TTR will be uploaded in 4Q 2025.